Metering or dosing pumps are devices capable of delivery of predetermined quantities of material per unit of time or per unit of the operation of an activating system, and may be used in a wise variety of material feed applications.
For example, the fuel-injection pump of a diesel engine can be considered a metering or dosing pump as defined above because predetermined quantities of the diesel fuel are delivered per stroke of the pump.
Such pumps also have application in the food industry, in the pharmaceutical field, in chemical processing and in the synthetic resin field, as well as in industry and research generally. Indeed, such pumps are commercially available in a variety of configurations with various drives and various capacities.
In one class of metering pump, a reciprocating drive member serves to displace the material. In this system a chamber is provided and communicates via an intake or suction valve with a source of the material to be dispensed, e.g. a reservoir containing the material under pressure, under a hydrostatic head or under no pressure, the chamber also being connected via an outlet or discharge valve with a pump outlet which is preferably fitted with a nozzle, i.e. a small-cross section elongated discharge passage.
Upon the retraction stroke, the drive member is shifted in a direction tending to expand the chamber and material from the reservoir is induced to flow through the unidirectional or check valve at the intake into the chamber while the unidirectional or check valve at the discharge prevents retraction of material from the outlet passage and nozzle.
Upon the displacement stroke of the drive member the volume of the chamber is reduced and the fluid within the chamber is forced past the discharge valve, the presssure within the chamber developed by advance of the drive member blocking the intake valve.
Cycling in this manner with the stroke of the drive member precisely determined, feeds a predetermined quantity of the material from the outlet with each forward stroke of the drive member which can be a piston (plunger) or membrane.
The drive member itself can be displaced by fluid pressure applied to a side thereof opposite the side exposed to the material which is displaced.
While such pumps are widely used and in diverse fields of application, problems are encountered when the material is or contains a volatile substance. The term "volatile substance" is here used to describe any material which has an extremely high vapor pressure at the temperature at which the pump is to operate so that a surface of the material exposed to the atmosphere will suffer a change because of evaporation of the volatile substance. Such volatile substances can be fuels but most often are solvents forming vehicles for other materials.
Hence upon evaporation of the volatile substance with exposure to the atmosphere a change in the composition of the material can occur.
Since the outlet of the pump is generally exposed to the atmosphere, and the displacement of the material by the drive member brings the material to the very end of the outlet where it may be exposed to air and evaporation of the volatile substance can occur, various problems result. For example, because of loss of the solvent at the tip of the outlet nozzle, the concentration of other materials will increase and the viscosity will change. Both the change in viscosity and composition are generally detrimental to sensitive operations.
With certain materials, e.g. synthetic resins, a skin or film may harden at the exposed air/material interface so that obstruction of the pump can occur or further displacement may carry a heterogeneous product into any collecting passage or chamber. This problem is most pronounced where relatively long interludes separate the feed strokes of the pump so that contamination or evaporation at the interface is especially high.